Phased array multibeam formation antenna system



May 13, 1969 w. w. MUMFORD PHASED ARRAY MULTIBEAM FORMATION ANTENNA SYSTEM Filed March 4, 1968 PHASED ARRAY RADAR RECEIVER INCLUDING LOCAL O$C/LLA7'OR,DATA PROCE$$OR,BEAM RECEIVERS DATA PROCESSOR (BEAM FORM/N6 a STEERING) BEAM RECEIVERS lNl ENTOR W. W. MUMFORD BY ATTORNEY FIG? Unite States 3,444,559 PHASED ARRAY MULTIBEAM FORMATION ANTENNA SYSTEM William W. Mumford, Parsippany-Troy Hills Township,

Morris County, N.J., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill, N.J., a corporation of New York Filed Mar. 4, 1968, Ser. No. 709,977 Int. Cl. H04b 7/06 US. Cl. 343-100 8 Claims ABSTRACT OF THE DISCLOSURE A beam forming means for a multibeam phased array radar receiver in which the local oscillator beating frequency is space fed from the receiver to the rear of the antenna array. A frequency converter in each of the array elements down converts the received radar signals to corresponding signals at a lower intermediate frequency band. A coaxial cable from each array element transmits these lower frequency signals to the data processor.

Government contract The invention herein claimed was made in the course of, or under contract with the Department of the Army.

Background the invention This invention relates to the art of radio communications and more particularly to a phased array antenna system capable of simultaneously forming a separate beam for each of a plurality of signal sources in space.

Phased array antenna systems have been known for some time and the basic principles, particularly for linear arrays, are quite old as disclosed in U .8. Patent 2,286,839 granted June 16, 1942, to S. A. Schelkunoff. This patent describes a directive linear array antenna system and cites other art related to the same subject. Two-dimensional planar arrays for radar applications have been developed from these principles and, because of the number of antenna elements involved, the circuitry leading to them has become quite complicated and costly. This complication and cost have become immensely aggravated by the need to simultaneously form a plurality of beams. Multibeam phased array radars generally accomplish the beam forming function remote from the antenna proper in what is currently a rather massive structure located in the main building housing the radar. The elements of the antenna are connected to the beam forming networks by a circuit structure known as a corporate feed which comprises lengthy coaxial cables constructed closely as to length and phase stability. Also, in the beam forming network, additional waveguides and coaxial lines are used in achieving the summation of the signals from the antenna elements. These elaborate cables and formation networks contribute significantly to the high cost of multifunction array radars. Applicant is aware that single function array radar systems exist which use a space feed to good advantage and accomplish the phase control required for each antenna element through a phase shifter incorporated immediately behind the antenna element.

In an earlier copending application, Ser. No. 598,781, now U.S. Patent No. 3,406,399, filed Dec. 2, 1966, by D. A. Alsberg and assigned to the same assignee as the present application, a phased array system is disclosed which greatly simplifies the feed structure. That inven tion embodies a frequency multiplex principle in which local oscillators, one for each beam to be formed, transmits beating frequencies from a common space feed antenna through space to the antenna array. Modulators and other circuitry providing frequency separation funcice tions are included in each of the antenna elements to cooperate with the local oscillators to form the required beams. The modulation products are space fed back to the common antenna. This results in the complete elimination of all the conventional corporate feed structures with consequent simplification and economy over the corporate feed structures of the prior art.

Another copending application, Ser. No. 616,577 filed Feb. 16, 1967, by D. A. Alsberg and R. D. Tuminaro, also assigned to the same assignee as the present application, describes an invention which embodies a different principle called space multiplexing as distinguished from the frequency multiplexing principle of the earlier application. This later application, which eliminates the local oscillators and modulators, has a feed horn and a phase control means in each antenna element of the array so that radar energy received by the element may be radiated with controlled phase to a plurality of receiving horns located behind the array. In this way, the energy received by the array from all the sources in space can be simultaneously brought to a focus at different selected ones of the receiving horns at the rear of the array.

Summary 0 the invention The present invention comprises a multibeam phased array receiving antenna in which an array of receiving elements simultaneously receive microwave signals from a plurality of sources. Each receiving element comprises a receiving antenna on the front face of the array to receive the signals, an internal space feed antenna on the rear face of the array to receive a local oscillator heating frequency and a down converter coupled between these two antennas to derive lower intermediate frequency signals from the signal and local oscillator frequencies. The intermediate frequency signals are returned to the data processors via transmission lines, e.g., coaxial cables, for beam forming and steering in the data processors.

As only one local oscillator is used, this invention is much simpler than the invention of the copending Alsberg application and also simplifies the isolation problems between signals of simultaneously existing beams. While phase stability problems become almost impossible to solve when transmitting signals over coaxial cables at the high radar frequencies, they become quite manageable at the lower intermediate frequencies. Moreover, by eliminating the need for the plurality of receiving horns behind the antenna array, the present invention also offers considerable simplification over the copending Alsberg- Tuminaro application. In addition, thenumber of beams that can be formed by the earlier invention is limited by the number of receiving horns and beating oscillator frequencies whereas the number of beams that may be formed by the present invention is practically unlimited.

Brief description of the drawings The invention may be better understood by reference to the accompanying drawings in which:

FIG. 1 is a block diagram of an antenna system illustrating the salient features of this invention;

FIG. 2 shows the essential circuit details in accordance with one embodiment of the invention; and

FIG. 3 is a fragmentary circuit diagram replacing a portion of FIG. 2 to illustrate another embodiment of the invention.

Detailed description FIG. 1 discloses an antenna system comprising an array 1 of a plurality of receiving elements 2, each element having a receiving antenna 3 on its front face for receiving signal energy from a plurality of sources located at random points in space in front of the antenna. Each of the receiving elements also has an internal space feed antenna 4 located on its rear face and the entire array 1 is positioned in front of a radar receiver 7 having a space feed horn 6 arranged for transmitting local oscillator beating frequency energy through the interven ing space in the direction of arrow 9 to all of the internal space feed antennas 4. As is customary with space feed systems, the intervening space 5 is usually enclosed within a chamber, not shown, the walls of which are capable of absorbing substantially all stray energy, thereby permitting no appreciable amount of reflected energy to reach the rear face of the array.

The radar receiver 7 contains conventional phased array radar receiver equipment including a local oscillator coupled to the space feed horn 6, data processor equipment and beam receivers. Each of the receiving elements 2 contains a frequency converter coupled between its receiving and internal space feed antennas so that energy received by the two antennas may beat together, thereby down converting to an intermediate frequency level. The intermediate frequency outputs of these converters are carried back to the receiver 7 by way of coaxial cables, these cables being indicated in FIG. 1 by reference numeral 8. Other types of transmission lines may be used in place of the coaxial cables.

By transmitting the high frequency local oscillator energy through space 5, phase stability difficulties, due to temperature variations, are virtually eliminated. A high degree of isolation between the down converted signals coming from the several frequency converters in the array is achieved by using a separate coaxial cable for each one. Since these signals are at a low intermediate frequency, phase stability problems, due primarily to temperature variations, are substantially reduced. It is thus apparent that this unique combination of circuit elements provides a highly simplified multifunction array radar antenna system with inherent optimum stability.

FIG. 2 shows the details of the circuitry within antenna array 1 and the essential circuitry in the radar receiver 7. This figure shows a fragmentary cross-section of a portion of the array 1 showing three of the receiving elements 2. Each of these elements comprises a receiving antenna 3 on its front face and an internal space feed antenna 4 at its rear face. These two antennas are coupled together through a down converter so that signal energy received at the radar frequency by the receiving antenna 3 may be combined with the local oscillator energy 9 received by the internal space feed antenna 4 to produce a lower intermediate frequency. Amplifiers, filters and limiters, not shown, may also be included in the antenna elements, depending upon system requirements. This lower intermediate frequency is carried over one of the coaxial cables 8 to the radar receiver unit 7. Only three of these coaxial cables 8A, 8B and 8N are shown in FIG. 2 but it is understood that a separate cable such as 8A is provided for each of the receiving elements 2 contained in the array.

By reason of the fact that the path lengths from the space feed horn 6 to the several internal space feed antennas 4 are all different, an initial beam alignment adjustment will be required. This adjustment is conveniently provided by the phase adjusting means 21, which may be a phase shifter or delay line, included between each receiving antenna 3 and its internal space feed antenna 4. Alternatively, this adjustment can be provided by the beam forming and steering equipment. The manner by which this adjustment is made is well known in the phased array radar art and requires no further description.

The circuitry within the radar receiver 7 includes an amplifier 75 connected to each of the coaxial cables, the output from which is applied to one input terminal of an up converter 74. The other input terminal of the up converter 74 is coupled to the local oscillator 70 by way of circuit paths 71, 72 and 73. Local oscillator 70 also supplies energy to the space feed horn 6 by way of coupling path 71 so that the same frequency used for down converting the radar signals received by receiving antennas 3 is used for up converting the intermediate frequency energy carried back to the radar receiver 7 over cables 8. The outputs from the several up converters 74 are carried by way of coaxial cables 76 to the nearby data processor, which includes the beam forming and steering equipment, and beam receivers 77, all of which are of conventional construction well known in the phased array radar art. Since each receiving element in array 1 has its own coaxial cable, there will be as many coaxial cables 76 as there are receiving elements 2 in array 1. The conventional phase adjusting means in the data processor may, therefore, take the energy from the several coaxial cables and form as many beams as are required, steering each one to direct it toward a selected source in front of the array 1.

Instead of up converting the intermediate frequency energy in radar receiver 7, the intermediate frequency energy emerging from the several amplifiers 75 may be supplied to the data processor by a power divider 79 as shown in FIG. 3. Power divider 79 provides as many branches 80 to the data processor 77 as there are beams to be formed and steered.

What is claimed is:

1. An antenna system for a multibeam phased array receiver comprising an array of receiving elements for receiving signal energy from a plurality of sources located in space in front of said elements, each of said elements having a receiving antenna on its front face for receiving said signal energy, an internal space feed antenna at the rear of each element, a frequency converter in each element connected to its receiving and space feed antennas to convert the signals to lower intermediate frequency signals, a radar receiver including a local oscillator, a space feed horn coupled to said oscillator and positioned to transmit energy from said local oscillator to all of said internal space feed antennas, a data processor in said radar receiver and a coaxial line coupling each of said frequency converters to said data processor for transmitting the lower intermediate frequency signals to said data processor for beam forming and steering.

2. The combination of claim 1 wherein said coaxial line from each converter is coupled to said data processor by way of a power divider which divides the power from said line into as many branches as there are beams to be formed and steered by said data processor.

3. The combination of claim 1 wherein said coaxial line from each converter is coupled to said data processor by way of a frequency converter in said radar receiver, said frequency converter being coupled to said local oscillator to up convert the intermediate frequency signals in the coaxial line to the original signal frequency before transmitting them to said data processor.

4. The combination of claim 1 and a phase adjusting means connected between said receiving and space feed antennas in each of said receiving elements to provide an initial phase adjustment for initial beam alignment.

5. An antenna system for a multibeam phased array receiver comprising an array of receiving elements, each of said elements having a receiving antenna on its front face and a space feed antenna on its rear face, a frequency converter in each element coupled between said antennas for down converting signals simultaneously received by the receiving antenna from a plurality of sources, a radar receiver containing a local oscillator, a space feed horn coupled to said oscillator, a data processor and beam receivers, said system being particularly characterized by having said space feed horn positioned to transmit energy from said local oscillator to all of said space feed antennas and a transmission line coupling the frequency converter in each receiving element to the data porcessors in said radar receiver for transmitting the down converted signals to the data processor to simultaneously form and steer a plurality of beams corresponding to said plurality of sources.

6. The combination of claim 5 wherein said transmission line is coupled to said data processor by way of a power divider which divides the power from said cable into as many branches as there are beams to be formed and steered by said data processor.

7. The combination of claim 5 wherein said transmission line is coupled to said data processor by way of a frequency converter in said radar receiver, said frequency converter being coupled to said local oscillator to up convert the intermediate frequency signals in the transmission line to the original signal frequency before transmitting them to said data processor.

8. The combination of claim 5 and a phase adjusting means coupled between said receiving and space feed antennas in each of said receiving elements to provide an initial phase adjustment for initial beam alignment.

References Cited UNITED STATES PATENTS 3,406,399 10/1968 Alsberg 343100X RODNEY D. BENNETT, IR., Primary Examiner.

10 T. H. TUBBESING, Assistant Examiner.

US. Cl. X.R. 

